U.S. patent number 7,448,269 [Application Number 11/244,747] was granted by the patent office on 2008-11-11 for scanning near field ultrasound holography.
This patent grant is currently assigned to Northwestern University. Invention is credited to Vinayak P. Dravid, Gajendra Shekhawat.
United States Patent |
7,448,269 |
Shekhawat , et al. |
November 11, 2008 |
Scanning near field ultrasound holography
Abstract
A high spatial resolution phase-sensitive technique employs a
scanning near field ultrasound holography (SNFUH) methodology for
imaging elastic as well as viscoelastic variations across a sample
surface. SNFUH uses a near-field approach to measure time-resolved
variations in ultrasonic oscillations at a sample surface. As such,
it overcomes the spatial resolution limitations of conventional
phase-resolved acoustic microscopy (i.e. holography) by eliminating
the need for far-field acoustic lenses.
Inventors: |
Shekhawat; Gajendra (Arlington
Heights, IL), Dravid; Vinayak P. (Glenview, IL) |
Assignee: |
Northwestern University
(Evanston, IL)
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Family
ID: |
46322849 |
Appl.
No.: |
11/244,747 |
Filed: |
October 6, 2005 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20060037401 A1 |
Feb 23, 2006 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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10913086 |
Aug 6, 2004 |
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60494532 |
Aug 12, 2003 |
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Current U.S.
Class: |
73/603; 181/101;
73/596 |
Current CPC
Class: |
G01N
29/0663 (20130101); G01N 29/0681 (20130101); G01N
29/069 (20130101); G01N 29/265 (20130101); G03H
3/00 (20130101); G01Q 60/32 (20130101); G01N
2291/0232 (20130101); G01N 2291/02827 (20130101); G01N
2291/0427 (20130101) |
Current International
Class: |
G01N
29/04 (20060101); G01V 1/00 (20060101) |
Field of
Search: |
;73/105,603,584,596
;250/306,307 ;331/15,23,41,42,107 ;181/101 ;367/87 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Cuberes et al., Heterodyne Force Microscopy: Nanomapping of
Viscoelastic Response at Ultrasonic Speeds, Abstract of STM '99
conference, Jul. 1999, Seoul, Korea. cited by other .
Kolosov et al., Ultrasonic Force Microscopies--merging ultrasound
and SPM to explore nanometre scale mechanics on the nanosecond time
scale, Extended Abstract of STM '99 conference, Jul. 1999, Seoul,
Korea. cited by other .
Cuberes et al., Heterodyne force microscopy of PMMA/rubber
nanocomposites: nanomapping of viscoelastic response at ultrasonic
frequencies, J. Phys. D: Appl. Phys. 33, 2000, pages 2347-2355, IOP
Publishing Ltd. cited by other .
Geer et al., Nanometer-scale mechanical imaging of aluminum
damascene interconnect structures in a low-dielectric-constant
polymer, Journal of Applied Physics, Apr. 1, 2002, pages 4549-4555,
vol. 91, No. 7. cited by other .
Kolosov, UFM shakes out the details at the nanoscopic scale,
Materials World, Dec. 1998, pages 753-754. cited by other.
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Primary Examiner: Williams; Hezron E.
Assistant Examiner: Saint-Surin; Jacques M.
Attorney, Agent or Firm: McAndrews, Held & Malloy,
Ltd.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of non-provisional
application Ser. No. 10/913,086 filed Aug. 6, 2004 now abandoned,
which claims priority to provisional application Ser. No.
60/494,532 filed Aug. 12, 2003, which are hereby incorporated
herein by reference in their entirety.
Claims
The invention claimed is:
1. An atomic force microscopy system, said system comprising: a
cantilever, wherein said cantilever includes a tip at an end of
said cantilever; a vibrating device for supplying vibrations at a
first frequency to said cantilever to generate vibration at said
tip; and a detector for detecting movement of said tip based on an
atomic force between said tip and a surface of a sample, wherein
said sample is vibrated at a second high frequency excitation
different from said first frequency, wherein said first frequency
vibration and said second frequency vibration are mixed together to
form a mixed acoustic wave signal, and wherein said mixed acoustic
wave signal is processed electronically to determine an internal
microstructure of said sample based on phase and amplitude of said
mixed acoustic wave signal.
2. The system of claim 1, further comprising an electronic feedback
circuit configured to maintain vibration of said cantilever tip at
a resonance frequency.
3. The system of claim 2, wherein said electronic feedback circuit
comprises a MOSFET-based electronic readout circuit.
4. The system of claim 1, wherein said first frequency and said
second frequency comprise carrier frequencies which are amplitude
modulated individually.
5. The system of claim 4, wherein at least one of product
frequencies, beat frequencies, and sum frequencies are used to
determine said first and second carrier frequencies.
6. The system of claim 1, wherein said cantilever operates in near
contact mode without contact between said tip and said sample.
7. The system of claim 6, wherein at least one of beat frequencies,
product frequencies, and frequency addition are used to monitor
said sample in near contact mode.
8. The system of claim 7, wherein at least one of harmonics and
fundamental frequencies are used in generating said at least one of
beat frequencies, product frequencies, and frequency addition.
9. The system of claim 1, wherein said movement of said tip is used
to identify at least one of buried nanostructures, defects, and
dopant mapping in said sample.
10. The system of claim 1, wherein said movement comprises a linear
interaction between said tip and said sample.
11. A scanning near field acoustic holography system, said system
comprising: an acoustic wave generator configured to launch a first
high frequency acoustic wave from the bottom of a specimen and a
second acoustic wave from the base of a cantilever, wherein said
cantilever includes a tip at one end, and wherein said tip serves
as an antenna receiving phase and amplitude information from an
acoustic signal; and a scanning near field acoustic holography
module (SNFUH) electronic module capable of mixing the first and
second acoustic waves to generate at least one of a product
frequency, an addition frequency, and a difference frequency
representative of a surface and subsurface of said specimen.
12. The system of claim 11, wherein said electronic module further
comprises feedback electronics capable of providing feedback to
maintain said first and second acoustic waves.
13. The system of claim 11, wherein said first and second acoustic
waves comprise fundamental resonance frequencies and related
harmonics.
14. The system of claim 11, wherein said first acoustic wave is
perturbed by features beneath the surface of said specimen.
15. The system of claim 14, wherein said electronic module detects
perturbation of said first acoustic wave and constructs a
representation of the wave perturbation indicating the internal
microstructure of said specimen.
Description
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
N/A
BACKGROUND OF THE INVENTION
Known acoustic microscopes are used for imaging structures such as
integrated circuit (IC) structures. The spatial resolution, w, of
an acoustic microscope is given by:
.times..times. ##EQU00001## where .theta. is the speed of sound in
the coupling medium, f is the frequency of the acoustic/ultrasonic
wave, and N.A. is the numerical aperture of the lens. For a
frequency of 1 GHz, the nominal spatial resolution attainable is
approximately 1.5 .mu.m. Further, the acoustic microscope has two
other major roadblocks in getting high resolution: (1) impedance
mismatches and coupling fluid attenuation that is proportional to
f.sup.2. Higher resolution alternatives for nondestructive
mechanical imaging include the atomic force microscope (AFM) or
scanning probe microscope (SPM) platforms. A few examples include:
force modulation microscopy (FMM) as described by P. Maivald, H. J.
Butt, S. A. C. Gould, C. B. Prater, B. Drake, J. A. Gurley, V. B.
Elings, and P. K. Hansma in Nanotechnology 2, 103 (1991);
ultrasonic-AFM as described by U. Rabe and W. Arnold in Appl. Phys.
Lett. 64, 1423 (1994); and ultrasonic force microscopy (UFM) as
described by O. V. Kolosov, K. Yamanaka in Jpn. J. Appl. Phys. 32,
1095 (1993); by G. S. Shekhawat, O. V. Kolosov, G. A. D. Briggs, E.
O. Shaffer, S. Martin and R. Geer in Nanoscale Elastic Imaging of
Aluminum/Low-k Dielectric Interconnect Structures, presented at the
Material Research Society, Symposium D, April 2000 and published in
Materials Research Society Symposium Proceedings, Vol. 612 (2001)
pp. 1.; by G. S. Shekhawat, G. A. D. Briggs, O. V. Kolosov, and R.
E. Geer in Nanoscale elastic imaging and mechanical modulus
measurements of aluminum/low-k dielectric interconnect structures,
Proceedings of the International Conference on Characterization and
Metrology for ULSI Technology, AIP Conference Proceedings. (2001)
pp. 449; by G. S. Shekhawat, O. V. Kolosov, G. A. D. Briggs, E. O.
Shaffer, S. J. Martin, R. E. Geer in Proceedings of the IEEE
International Interconnect Technology Conference, 96-98, 2000; by
K. Yamanaka and H. Ogiao in Applied Physics Letters 64 (2), 1994;
by K. Yamanaka, Y. Maruyama, T. Tsuji in Applied Physics Letters 78
(13), 2001; and by K. B. Crozier, G. G. Yaralioglu, F. L.
Degertekin, J. D. Adams, S. C. Minne, and C. F. Quate in Applied
Physics Letters 76 (14), 2000. Each of these techniques is
traditionally sensitive to the static elastic properties of the
sample surface.
Recent developments in atomic force microscopes have involved the
application of ultrasonic frequency (MHz) vibrations to the sample
under study and non-linearly detecting of the deflection amplitude
of the tip at the same high frequencies. With this arrangement,
which is commonly identified as an ultrasonic force microscope, the
ultrasonic frequencies employed are much higher than the resonant
frequency of the microscope cantilever. The microscope exploits the
strongly non-linear dependence of the atomic force on the distance
between the tip and the sample surface. Due to this non-linearity,
when the surface of the sample is excited by an ultrasonic wave,
the contact between the tip and the surface rectifies the
ultrasonic vibration, with the cantilever on which the tip is
mounted being dynamically rigid to the ultrasonic vibration. The
ultrasonic force microscope enables the imaging and mapping of the
dynamic surface viscoelastic properties of a sample and hence
elastic and adhesion phenomenon as well as local material
composition which otherwise would not be visible using standard
techniques at nanoscale resolution.
The drawback of ultrasonic microscopy is that it measures only the
amplitude due to ultrasonically induced cantilever vibrations.
Moreover, where the sample is particularly thick and has a very
irregular surface or high ultrasonic attenuation, only low surface
vibration amplitude may be generated. In such circumstances the
amplitude of vibration may be below the sensitivity threshold of
the microscope in which case measurement is impossible. Moreover,
none of the above mentioned techniques measures with high
resolution the acoustic phase, which is very sensitive to
subsurface elastic imaging and deep defects identification which
are lying underneath the surface, without doing any cross
sectioning of the samples.
Out-of-plane vibrations created by non-linear tip sample
interaction make a very hard elastic contact with the sample
surface. Ultrasonic force microscopy (UFM) uses the same method
except for a amplitude component rather than a phase contrast. If
non-linearity is present in the system, most of the phase contrast
will come from the surface and not from a surface/sub-surface phase
contrast. Additionally, non-linear tip sample interaction may not
provide results for soft materials. Furthermore, in UFM, high
mechanical contrast may be acquired with little sub-surface
contrast.
BRIEF SUMMARY OF THE INVENTION
The present invention relates to a high spatial resolution
phase-sensitive technique, which employs a scanning near field
ultrasonic holography methodology for imaging buried or other
subsurface structures or variation in the specimen. Scanning near
field ultrasound holography (SNFUH) uses a near-field approach to
measure time-resolved variations in ultrasonic oscillations at a
sample surface. As such, it overcomes the spatial resolution
limitations of conventional phase-resolved acoustic microscopy
(i.e. holography) by eliminating the need for far-field acoustic
lenses.
The fundamental static and dynamic nanomechanical imaging modes for
the instrument of the present invention are based on nanoscale
viscoelastic surface and subsurface (e.g., buried nanostructure)
imaging using two-frequency ultrasonic holography. The scanning
near-field ultrasonic technique of the present invention vibrates
both the cantilevered tip and the sample at ultrasonic/microwave
frequencies. The contact, soft-contact and near-contact modes of
tip-sample interaction enable the extraction of the surface
acoustic waves signal between the two ultrasonic vibrations.
Perturbations to the phase and amplitude of the surface standing
acoustic wave may be locally monitored by the SPM acoustic antenna
via lock-in and SNFUH electronic module. As the specimen acoustic
wave gets perturbed by buried features, the resultant alteration in
the surface acoustic standing wave, especially its phase, is
effectively monitored by the SPM cantilever. Thus, within the
near-field regime (which enjoys superb spatial resolution), the
acoustic wave (which is non-destructive and sensitive to
mechanical/elastic variation along its path) is fully analyzed,
point-by-point, by the SPM acoustic antenna in terms of its phase
and amplitude. Thus, as the specimen is scanned across, a pictorial
representation of specimen acoustic wave's perturbation is recorded
and displayed, to offer quantitative account of the internal
features of the specimen.
Certain embodiments provide contact, soft (e.g., intermittent)
contact, and/or near contact modes of operation to identify surface
and subsurface (e.g., buried) characteristics of a specimen.
Additionally, an SNFUH electronic module extracts surface acoustic
phase and amplitude with or without non-linear tip sample
interaction
These and other advantages and novel features of the present
invention, as well as details of an illustrated embodiment thereof,
will be more fully understood from the following description and
drawings.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
FIG. 1 is a block diagram illustrating the scanning probe
microscope with scanning near field ultrasound holography of the
present invention;
FIG. 2 is an illustration of atomic force microscopy of the present
invention with a vibrating cantilever tip and vibrating sample;
FIG. 3(A) is a schematic illustration of a model nanoparticle
system for validation of SNFUH.
FIG. 3(B) shows an AFM (topography) image with a featureless top
polymer surface.
FIG. 3(C) shows the phase image of SNFUH revealing buried gold
nanoparticles with high definition.
FIG. 4(A) shows a schematic of a model test sample for detecting
embedded defects/voiding in shallow trenches.
FIG. 4(B) shows an AFM (topography) image with a uniform coating of
dielectric material.
FIG. 4(C) shows a phase image of SNFUH that reveals the surface
elastic contrast and embedded voiding in polymer coating over
nitride and hardening of the coating at the trench walls.
FIG. 4(D) shows a line profile across a void marked across X-Y.
FIG. 5(A) shows an AFM topography of malaria-infected red blood
cells.
FIG. 5(B) shows an SNFUH phase image from malaria-infected red
blood cells.
FIG. 5(C) represents an AFM topography of early-stage incubation of
parasite infection in malaria-infected red blood cells.
FIG. 5(D) represents an SNFUH phase image of early-stage incubation
of parasite infection in malaria-infected red blood cells.
FIG. 6(A) depicts AFM (topography) imaging of a copper-low K
dielectric interconnect system.
FIG. 6(B) depicts SNFUH imaging of a copper-low K dielectric
interconnect system.
FIG. 6(C) shows a line profile across the voids in FIG. 6(B).
FIG. 7 shows a feedback control circuit used in accordance with an
embodiment of the preset invention.
FIG. 8 illustrates the feedback circuit in the context of an
embodiment of an electronic readout system used in accordance with
an embodiment of the present invention.
FIG. 9 illustrates a flow diagram for a method for scanning near
field acoustic imaging used in accordance with an embodiment of the
present invention.
The foregoing summary, as well as the following detailed
description of certain embodiments of the present invention, will
be better understood when read in conjunction with the appended
drawings. For the purpose of illustrating the invention, certain
embodiments are shown in the drawings. It should be understood,
however, that the present invention is not limited to the
arrangements and instrumentality shown in the attached
drawings.
DETAILED DESCRIPTION OF THE INVENTION
Certain embodiments of the present invention are directed to a
nondestructive, high resolution, sub-surface nanomechanical imaging
system. The system is capable of directly and quantitatively
imaging the elastic (static) and viscoelastic (dynamic) response of
a variety of nanoscale materials and device structures with spatial
resolution of a few nanometers depending on the ultrasonic
frequencies. For viscoelastic high resolution sub-surface
nanomechanical imaging the target maximum probe frequency is around
5-10 GHz, for example. In an embodiment, the maximum relative phase
resolution at this frequency is estimated to be 0.001.degree.
leading to a viscoelastic time resolution of less than <1 ps.
The instrument of certain embodiments of the present invention
operates in a manner similar to commercially available scanning
probe microscopes (SPMs) in that quantitative, digital, rastered,
nanometer-scale images are obtained of the sample elastic modulus,
and sample viscoelastic response frequency. The instrument also
provides conventional SPM imaging modes including topography,
frictional, and force modulation imaging.
Applications for certain embodiments of the present invention are
numerous and represent areas of critical need in Molecular
electronics, Nanosystems (NEMS), and Nanotechnology, in general. By
combining the nanometer-scale spatial resolution of conventional
SPMs with the sub-surface defect identification and imaging
capabilities of acoustic or ultrasonic microscopes, the instrument
fills a critical need in characterizing and investigating the
nanomechanics of nanoscale systems. The SNFUH system and method may
be used for: (1) in-vitro imaging of biological specimens, tissues
and cells, (2) nanomechanical imaging of buried structures,
inclusions in nanocomposites, failure analysis in IC structures and
devices, (3) mechanical properties of low-K materials, (4) stress
variation in 3D structures and interconnects, (5) flaw imaging in
ceramics and quantitative evaluation of mechanical properties,
etc.
Certain embodiments of the present invention are based on Nanoscale
viscoelastic surface and subsurface (e.g., buried nanostructure)
imaging using two-frequency ultrasonic holography. This is
essentially a `scanning near-field` ultrasound technique, where
both the cantilevered tip 10 and the sample 12 are vibrated at
ultrasonic/microwave frequencies. Contact and soft-contact
tip-sample interaction enables the extraction of surface acoustic
wave amplitude and phase with high resolution.
In SNFUH mode, perturbation to the standing surface acoustic wave
resulting from specimen acoustic wave scattering is monitored by an
SPM acoustic antenna. The resulting cantilever deflection merely
follows the perturbation to the surface standing acoustic wave,
which represents the dissipative lag/lead in the surface response
with respect to the tip reference frequency (i.e. the time of
flight delay of the specimen acoustic waves reaching the sample
surface). Extracting the spatial dependence of this phase term
provides image contrast indicative of the relative elastic response
of the buried structures, interfaces, and embedded defects to the
specimen acoustic wave and thus the resultant perturbation to the
standing surface acoustic wave.
Certain embodiments of the present invention provide a system that
measures subsurface (e.g., buried) defects, delaminations; cracks;
stress migration and etc., while maintaining the high resolution of
the atomic force microscope. It utilizes (1) an atomic force
microscopy system having a cantilever 14 with a tip 10 at a free
end sitting on top of the vibrating device 16 for supplying
vibrations to the cantilever at a frequency greater than cantilever
resonance frequency, (2) a sample 12 having a vibration device 18
sitting under it for providing high frequency excitations and (3)
an optical detector or other detector for detecting movement of the
cantilever. It detects the beats, products, additions frequencies
and beats, products of their harmonics and modulated waveforms,
when the vibrating tip interacts with the vibrating sample, which
falls within its detection range. With this embodiment, it is
possible to recover surface acoustic wave phase information of the
tip-surface mechanical interaction, which allows measurement of
viscoelastic properties and enables the application of acoustic
holography algorithms for imaging nanoscale sized sub-surface
(e.g., buried) defects. The microscopy apparatus utilizes scanning
Near Field Ultrasound Holography (e.g., SNFUH) for high resolution
nanomechanical imaging of buried defects and structures.
The surface acoustic wave's amplitude and phase are experimentally
extracted from the tip deflection signal via lock-in detection. The
phase sensitivity of this measurement is involved in extracting
time-resolved mechanical properties of materials as well as
potentially enabling subsurface imaging (e.g., buried
nanostructures).
Certain embodiments of the present invention detect the phase of
transmitted acoustic wave directly at wafer/device surface.
Further, certain embodiments of the present invention detect the
phase of surface acoustic wave directly at wafer/device surface.
Further, certain embodiments of the present invention utilize
scanning nanoprobe phase detection so as to eliminate the need for
acoustic lenses. The Nanoprobe Acoustic Antenna (AFM Tip) of
certain embodiments of the present invention is advantageous
because, for example, it provides induction of MHz-GHz nanoprobe
mechanical oscillations via high frequency flexural mode
excitation, i.e. the mechanical wave guide and the cantilever
monitors the phase shifts between tip 10 and sample 12
acoustic/ultrasonic vibrations.
As shown in FIGS. 1 and 2, two oscillations are applied to the tip
10 and sample 12 by two matched piezo crystals 16 and 18 attached
to the Si substrate of the tip and the base of the sample,
respectively. Each piezo 16, 18 is driven by a separate waveform
with a SNFUH electronic module 36 providing the input frequency to
an RF lockin amplifier 40 for surface acoustic wave (SAW) amplitude
and phase extraction. The SNFUH electronic module 36 selects beats,
product, and/or addition frequencies for example, to aid in
performing holography in contact, soft-contact, and near-contact
modes. Additionally, the SNFUH electronic module 36 allows SNFUH to
be performed in the linear regime of tip-sample interaction. In an
embodiment, the SNFUH electronic module 36 includes a mixer
circuit, variable resistor(s), op-amp(s), band pass filter(s),
and/or other filters for mixing frequency signals and selecting
frequency products, additions and beats, for example.
Any Scanning Probe Microscope (SPM) may serve as the base platform.
A signal access module (SAM) 22 is used as the input site for
SNFUH, and modulus-calibration signals. The integrated piezo (for
high frequency excitation) will enable ultrasonic excitation of
higher-order flexural resonances of the cantilever tip 10 to
provide the ultrasonic vibration.
The sample ultrasonic vibration is driven by function generator 32.
Second function generator 34 applies the sample ultrasonic
vibration. The resulting differential output signal from detector
is accessed with the signal access module (SAM) 22 and acts as the
input to RF lockin amplifier 30 or similar lockin amplifier for
extraction of SAW amplitude and phase. The lockin response signal
constitutes a SAW amplitude and phase which act as a input into the
signal acquisition electronics 46, via the SAM 22, for image
display and analysis. The SNFUH electronic module circuit 36
extracts beats, products of fundamental and harmonics, and/or
modulated waveform(s), which serve as a reference for a RF lockin
amplifier 40 or other lockin amplifier, for example. The
differential output of optical detector (A-B) is input, via the SAM
22, into the RF lockin 40. The resulting output constitutes the
SNFUH image signal. A computer 44 or other processor running data
acquisition/analysis software, such as Lab View or other data
acquisition and/or analysis software, acquires both the A-B signal
from the digital scope and the lockin. In an embodiment, a switch
may be included to select an SNFUH or UFM signal for acquisition,
for example.
In an embodiment, the sample piezo consists of an
insulator/electrode/piezo/electrode/insulator blanket multilayer
(e.g., 10 cm.times.10 cm) stack. The insulators consist of epoxied
machinable ceramics or thin, spin-cast polymer coatings, dependent
upon ultrasonic coupling efficiency. The Cr/Au electrodes or other
similar electrodes provide electrical contact between the piezo and
the second function generator 34. The assembly is counter-sunk into
a modified SPM sample mount.
As shown in FIG. 2, using SNFUH, a high frequency acoustic wave is
launched from below the specimen 12, while another high frequency
acoustic wave is launched at at least a slightly different
frequency is launched on the SPM cantilever 10. The SNFUH
electronic module 36 is used to spatially monitor the phase
perturbation to a standing surface acoustic wave, which results
from a scattered specimen acoustic wave. The resonant frequency of
the cantilever, f.sub.0, may be in the 10-100 kHz range, for
example.
Certain embodiments may also include a feedback circuit, such as
the cantilever resonance feedback circuit 50 depicted in FIG. 7.
The feedback circuit 50 includes a first op amp (OA) 52, a second
op amp (OA) 54, a phase compensator (PC) 56, a voltage-controlled
oscillator (VCO) 62, a waveform or function generator 68 and a
cantilever tip having a piezo transducer 74 interacting with a
sample 78.
For SNFUH operation to be uniquely calibrated across samples, phase
of the cantilever may be fixed. In order to fix the phase of the
tip, a resonance feedback circuit, said as feedback circuit 50 may
be employed. The feedback circuit 50 maintains the tip carrier
frequency at resonance and fixes or sets the phase, so that the tip
phase is a stable reference for sample phase. For example, at
higher frequencies (e.g., 150 MHz-10 GHz), the cantilever easily
strays from its resonance, and feedback may be used to maintain the
cantilevered tip frequency at resonance. Both the sample and
cantilever may maintain their resonance frequencies to generate a
high resolution viscoelastic response. In an embodiment, the system
may operate in feedback mode with the feedback circuit 50
generating feedback for frequency resonance, or the system may
operate without the feedback circuit 50 activated.
In the feedback circuit 50, the voltage-controlled oscillator 62
drives the tip piezo transducer 74. The VCO 62 is connected through
the phase compensator 56, which acts as an input to an op amp pair
52, 54 for feedback control. If the cantilever resonance frequency
shifts during scanning, the reduction in tip vibration amplitude
will reduce the voltage across the piezo transducer on the
cantilever. This voltage will cause a shift in the PC 56 output.
The shift in PC output will bring the VCO 62 back into
resonance.
FIG. 8 illustrates the feedback circuit 50 in the context of an
embodiment of an electronic readout system 800 used in accordance
with an embodiment of the present invention. The electronic readout
system 800 may be a MOSFET embedded electronic readout, for
example. Using embedded MOSFET as electronics feedback may provide
a current sensitivity of .DELTA.I.sub.d/I.sub.d=10.sup.-6/nm of
cantilever bending. Deflection sensitivity of the electronic
readout may be of the same order as optical feedback detection, for
example. In an embodiment, deflection sensitivity may be
approximately three orders of magnitude higher than existing
passive and active detection technologies, such as piezoresistive
detection. In an embodiment, a high signal-to-noise ratio and
minimal 1/f noise allow the MOSFET embedded electronic readout to
be used for electronic feedback in SPM's (scanning probe
microscopes), for example.
The feedback circuit 50 may be used to control a power supply 84
which supplies power to a piezo 86. The piezo 86 includes contacts,
such as Au (gold) contacts 88, as well as an actuator 90 and a
BiMOS transistor 92. The piezo 86 is driven by an oscillator 94.
Feedback from the oscillating piezo 86 is gathered by the
electronic detection unit 96. The feedback signal from the
electronic detection unit 96 is converted using the analog to
digital converter (ADC) 98 and fed into the feedback circuit 50 for
control of the power supply 84. Set point 100 provides a basis or
reference for operation of the feedback circuit 50. Feedback from
the circuit 50 helps to ensure that the tip and the sample are
being vibrated at their respective resonance frequencies, for
example.
An example of viscoelastic nanomechanical imaging is shown in FIG.
3. FIG. 3(A) shows gold nanoparticles dispersed on a polymer coated
substrate are buried under an approximately 500 nm thick polymer
layer. Use of a model polymer-nanoparticle composite demonstrates
the high lateral spatial resolution and depth sensitivity of the
SNFUH approach. A specimen consisting of gold nanoparticles buried
deep underneath a polymer cover layer was prepared by dispersing
colloidal gold nanoparticles on a polymer (poly
(2-vinylpyridine)-PVP) coated silicon substrate. The gold
nanoparticles have an average diameter of 15 nm and are well
dispersed on the film surface. The nanoparticles were then fully
covered with another polymer film about 500 nm thick, as shown
schematically in FIG. 3(A). The normal AFM topography scan, FIG.
3(B), shows a smooth featureless surface of top polymeric layer
with surface roughness of approximately 0.5 nm. On the other hand,
the phase image of SNFUH, FIG. 3(C), shows well-dispersed gold
nanoparticles buried approximately 500 nm deep from the top
surface. The contrast in the phase image of SNFUH arises from the
elastic modulus difference between the polymer and gold
nanoparticle, which induces the time dependent phase delay of the
acoustic waves reaching the sample surface.
To demonstrate the efficacy of SNFUH in identifying underlying
defects in narrower trenches, shallow trench structures may be
fabricated as shown in FIG. 4(A). The trenches are etched in SOD
(spin-on-dielectric) with a 50 nm thin layer of LPCVD
Si.sub.3N.sub.4 as a capping layer and then Si.sub.3N.sub.4 is
etched down in the 1 .mu.m deep trenches using the wet processing.
Trench width in the example is about 400 nm. A 500 nm thick layer
of polymer [Benzocyclobutene (BCB)] was spin-coated followed by
thermal annealing for curing the polymer.
FIG. 4(A) shows a schematic of series of isolated shallow trench
structures. FIG. 4(B) shows a conventional AFM topography image,
while FIG. 4(C) is a corresponding (simultaneously recorded) SNFUH
phase image. The typical 7.5.times.7.5 .mu.m.sup.2 topography scan
shows uniform and contiguous polymeric coating on SiN and inside
the trenches. On the other hand, the corresponding SNFUH phase
image shown in FIG. 4(B) reveals phase contrast reminiscent of
embedded voiding within the polymer, and at the SiN-polymer
interfaces. The dark contrast in the phase image in polymer coated
SiN lines corresponds to voids at polymer-SiN interface, i.e.
voiding underneath the contact. The contrast is due to the distinct
viscoelastic response from the specimen acoustic wave from the
voids, for example. A hardening of the polymer in the trench and
its sidewall is also evident in the phase image, which results from
thermal annealing and possibly poor adhesion with SOD, for example.
FIG. 4(D) shows a line profile of phase across X-Y from FIG. 4(C).
A subsurface phase resolution of 50 mdeg may be achieved, for
example. Current methods of diagnosis employ destructive approaches
such as wet etching followed by SEM imaging, which are undesirable.
Thus, SNFUH may be an improved tool-set for such subsurface
metrology needs.
The efficacy of SNFUH to imaging embedded or buried sub-structures
in biology is demonstrated in FIG. 5, which depicts high resolution
and remarkably high contrast arising from malaria parasites inside
infected red blood cells (RBCs). FIG. 5 demonstrates early stage
direct and real-space in-vitro imaging of the presence of parasites
inside RBCs without labels or sectioning of cells, and under
physiologically viable conditions. Plasmodium falciparum strain 3D7
was cultured in-vitro by a modification of the method of Haldar et
al. Parasites were synchronized to within 4 hours using a
combination of Percoll purification and sorbitol treatments,
cultured to 10% parasitemia, and harvested at the indicated times,
for example.
SNFUH imaging may be performed using the near-contact mode method
for imaging soft structures, for example. An SNFUH electronic
module may be used to bring the cantilever in near-contact mode and
then the sample was subsequently scanned over the RBCs while
maintaining the near-field regime. FIGS. 5(A) and 5(B) show AFM
topography images and SNFUH phase images from infected RBCs,
respectively. The AFM topography image shows the typical surface
morphology of infected RBC, while the SNFUH phase image shows high
contrast from the parasite residing well inside the RBC. In
addition to several other features reminiscent of membrane proteins
and sub-cellular contents, multiple parasites are clearly evident.
In order to further demonstrate the capability of SNFUH for early
stage diagnosis of parasite infection, RBCs incubated for only four
hours are examined, which is difficult to validate by other
non-invasive technique (e.g., fluorescence tagging). FIGS. 5(C) and
5(D) show a pair of images similar to those in FIGS. 5(A) and 5(B).
SNFUH may be sensitive to early stage parasite infection in RBC, as
reflected by image contrast consistent with parasite infection, for
example.
FIG. 6 shows a series of low-K dielectric polymer and copper lines
with lateral dimension of about 200 nm for the polymer and around
60 nm for copper. FIG. 6(A) shows the conventional topography
image, while FIG. 6(B) is the corresponding (simultaneously
recorded) SNFUH phase image. The typical 1400.times.1400 nm.sup.2
topography scan shows uniform and contiguous polymer and copper
lines. However, the corresponding SNFUH phase image shown in FIG.
6(B) reveals phase contrast reminiscent of sub-surface voiding in
copper lines. FIG. 6(C) shows a line profile across the voids. The
dark contrast in the phase image of copper lines corresponds to
voids underneath the metal. The presence of this contrast in phase
image implies that there is insufficient metal filling at the
bottom, i.e. voiding underneath the contact, which undergoes a
distinct viscoelastic response. Interestingly, a hardening of the
polymeric regions and its sidewall is also evident in the phase
image, which results from RIE processing and
chemical-mechanical-polishing (CMP). SNFUH may serve as a tool-set
for such sub-surface metrology challenges.
Thus, SNFUH may be used to facilitate: (1) quantitative high
resolution nanomechanical mapping of subsurface (e.g., buried)
structures to identify process-induced mechanical variations and/or
nanoscale cohesive defects; (2) nanomechanical viscoelastic
(dynamic) imaging to specifically investigate surface and
subsurface interfacial adhesive (bonding) response, etc.
Other applications for the system and method of the present
invention include: (1) non-destructive imaging of subsurface
defects in 3D interconnects and stress migration along the devices
due to electrical biasing; (2) non-destructive inspection for
interconnect nanotechnology for nanometer-scale resolution, to
enable imaging of electromechanical defects (e.g. nanotube
contacts) and to enable imaging of nanoscale integrity in molecular
interconnect assemblies; (3) subsurface nano-cracks, stress,
delamination identification in ferroelectrics, ceramics and
micromechanical structures and devices; (4) non-destructive defect
review and process control in integrated IC materials and devices
to provide modulus measurement for soft materials (i.e. porous
dielectrics) and to provide void and delamination defect detection
to avoid off-line, cross-sectional failure analysis; (5) self
assembled monolayers and subsurface defects in biological cells and
materials, in-vitro imaging of biological cells, tissues and
membranes, nano-bio mechanics and (6) quantitative extraction of
elastic modulus with high accuracy.
FIG. 9 illustrates a flow diagram for a method 900 for scanning
near field holography imaging used in accordance with an embodiment
of the present invention. First, at step 910, a sample is
positioned with respect to a cantilever for nanomechanical imaging
of the sample. Surface and/or sub-surface imaging may be performed
with respect to the sample, such as a tissue or other sample. At
step 920, a tip of the cantilever is vibrated at a first frequency.
The frequency may be a first microwave, ultrasonic, or other
acoustic frequency, for example. Then, at step 930, the sample is
vibrated at a second frequency, such as a second microwave,
ultrasonic, or acoustic frequency, for example. In an embodiment,
the second frequency vibrates the sample at a frequency that is
offset from the first frequency vibrating the tip. In an
embodiment, the tip and sample piezos are vibrated at their
respective resonance frequencies.
Next, at step 940, interaction between the vibrating tip and the
vibrating sample is detected. The interaction may be a physical
interaction and/or a non-contact signal interaction between the tip
and sample, for example. The interaction may constitute movement of
the tip, for example. Tip movement may be provided as a tip
deflection signal, for example. In an embodiment, the interaction
may include a linear and/or non-linear interaction between the tip
and the sample. In an embodiment, SNFUH may be performed using the
linear tip-sample interaction in soft contact and near contact mode
to obtain the high resolution sub-surface phase. SNFUH may be
performed in soft and near contact modes to obtain the sub-surface
information, such as buried defects or variations.
At step 950, amplitude and phase information associated with the
surface acoustic waves of the sample are extracted. Amplitude and
phase information may be extracted from the tip deflection signal
using lock in detection, for example. In an embodiment, subsurface
mechanical data, such as interfacial bonding, regarding the sample
may also be extracted from the tip deflection signal.
At step 960, surface and/or subsurface characteristics of the
sample may be imaged using the amplitude and phase information. In
an embodiment, a spatial variation of surface and subsurface
viscoelastic phase may be imaged, for example. In an embodiment, a
characteristic viscoelastic response time of the sample may be
defined based on the amplitude and phase information. Then, at step
970, vibration of the cantilever tip is maintained at a tip piezo
resonance frequency, and vibration of the sample is maintained at a
sample resonance frequency. Feedback, such as electrical feedback,
may be provided to maintain the tip resonance frequency and the
sample resonance frequency.
In an embodiment, product frequencies may be used with optical
detection to obtain biological imaging with high subsurface
resolution. The sample and cantilever are excited at their
fundamental resonance frequencies (e.g., 1.96 MHz and 3,28 MHz,
respectively). Additionally, the individual sample and cantilever
carriers are modulated with one or more modulation frequencies
(e.g., 25 kHz and 35 kHz, respectively). Next, a combination of a
SNFUH electronic module and RF lock in band pass filter outputs a
product of the two modulated waveforms. The product output is then
fed into a RF lock in amplifier reference input.
Using product frequencies allows improved selection of carrier
frequencies. In an embodiment, the larger the frequency of acoustic
oscillations, the higher the order of phase contrast that may be
obtained from SNFUH images. Thus, smaller features not seen at
lower carrier frequencies may be detected using a higher frequency
carrier. Additionally, use of product frequencies allows use of
non-matching tip and cantilever piezos.
In an embodiment, forces between the cantilever and sample may be
controlled during SNFUH operation in near contact mode. Contacting
the cantilever with the biological samples may rupture the samples.
However, near contact operation allows monitoring and subsurface
imaging of soft structures. Near contact mode operation may provide
sub-surface imaging of soft structures as well as providing
quantitative analysis of biological structures, cells and/or
tissues, for example.
In an embodiment, beat frequencies may be used to monitor samples
in near contact mode. Alternatively, frequency addition may be used
for sample monitoring in near contact mode. In an embodiment,
harmonics, as well as or in addition to fundamental frequencies,
may be used in beat frequency, product frequency, and/or frequency
addition (sum) operation. For example, the system may perform
optical and/or electronic detection according to a variety of
frequency strategies up to 1000 MHz with a very thin film of
ZnO.
In an embodiment, cantilever and sample carrier frequencies may be
modulated with amplitude modulation. For example, two carriers, a
carrier for the cantilever and a carrier for the sample are
amplitude modulated individually. In this configuration, the
tip-sample assembly may be excited with a higher frequency (with or
without matching piezos). Then, an amplitude modulated waveform may
be obtained from both the cantilever and sample and input to a
SNFUH electronic module. The output of the electronic module is a
product/difference/addition frequency. A beat or difference
frequency is a difference between modulation frequencies, for
example.
In an embodiment, an electronic readout device may be implemented
with the sample monitoring system. An example of such a readout
device may be the readout device described in U.S. patent
application Ser. No. 10/996,274, filed on Nov. 23, 2004, entitled
"Method and System for Electronic Detection of Mechanical
Perturbations Using BiMOS Readouts", which is herein incorporated
by reference. In an embodiment, using a readout circuit allows
product frequencies to be used without modulation since operation
is not limited by response time as with optical photodiodes, for
example.
In an embodiment, use of electronic detection eliminates optical
detection of amplitude and phase removes or eases limitations
imposed by a photo-detector response frequency, such as a 1 MHz
photo-detector response frequency. Electronic detection aids in
fabricating multi-active probes with on-chip integrated
piezo-actuator (e.g., ZnO) and embedded MOSFET feedback
electronics. Additionally, electronic detection does not limit
detection of subsurface features based on beat frequency. Multiples
of frequencies may be used to enhance both amplitude and subsurface
phase contrast, and thus the viscoelastic response. Enhanced
viscoelastic response results in enhanced phase contrast from
features less than 50 nm, for example, which may be difficult to
detect using only beat frequencies.
Thus, certain embodiments provide a scanning near field ultrasound
holography (SNFUH) technique to image high resolution buried
nanostructures, defects, 3D tomography, identification of
individual layers in multilayer thin film stacks and dopant
mapping, for example. Certain embodiments integrate three
approaches: a combination of scanning probe microscope platform
(which enjoys excellent lateral and vertical resolution) coupled to
micro-scale ultrasound source and detection (which facilitates
"looking" deeper into structures, section-by-section) and a
holography approach (to enhance phase resolution and phase coupling
in imaging). Certain embodiments provide near field, ultrasonic
holography, near field microwave holography, or other near field
acoustic holography for surface and subsurface imaging in nano- and
micro-specimens, such as biological, mechanical, and electrical
specimens. Certain embodiments allow SNFUH imaging using linear
and/or non-linear interactions between cantilever and specimen in
contact, soft contact and/or near contact mode, for example.
As a result, the technique allows subsurface flaw imaging in nano-
and micro-composites, MEMS, CMOS, and heterostructures, for
example. The technique also provides in-vitro imaging of
biopolymer, biomaterials and biological structures (e.g. viewing
cell-membrane or implant-bio interface). Additionally, certain
embodiments detect voiding and subsurface defects in low-K
dielectric materials and interconnects, as well as stress migration
and defect analysis in 3D interconnects and MEMS. Certain
embodiments facilitate dopant profiling and modulus mapping in
non-contact mode and also provide non-invasive monitoring of
molecular markers/tags -signal pathways, for example.
In an embodiment, a high frequency (e.g., on the order of hundreds
of MHz) acoustic wave is launched from the bottom of the specimen,
while another wave is launched on the AFM cantilever. These
acoustic waves are mixed together through a SNFUH electronic
module, which includes a combination of various filters, mixers,
feedback electronics and electronic components used to obtain a
desired product and addition of fundamental resonances and related
harmonics (in addition to difference frequencies). The resulting
mixed wave is monitored by the AFM tip, which itself acts as an
antenna for both phase and amplitude. As the specimen acoustic wave
gets perturbed by buried features, especially its phase, the local
surface acoustic waves are very effectively monitored by the AFM
tip. Thus, within the near-field regime (which enjoys superb
lateral and vertical resolution), the acoustic wave (which is
non-destructive and sensitive to mechanical/elastic variation in
its "path") is fully analyzed, point-by-point, by the AFM acoustic
antenna in terms of phase and amplitude. Thus, as the specimen is
scanned across, a pictorial representation of acoustic wave's
perturbation is fully recorded and displayed, to offer a
"quantitative" account of internal microstructure of the
specimen.
The SNFUH system is operational in the linear and near-contact
regime of tip-sample interaction and proves effective for in-vitro
imaging of biological cells and tissues using the SNFUH electronic
module, for example.
Thus, certain embodiments provide an electronic readout based on an
embedded MOSFET to detect product frequencies, which is thereby not
limited by an optical detector. Moreover, electronic readout may
help in building a parallel SNFUH system for industrial
application. In addition, a Brillion Zone Scattering technique may
be used to map the modules of any surface in non-destructive way
with greater efficiency than other methods.
Certain embodiments may be applied to microelectronics, especially
as an advanced nanoscale surface and sub-surface metrology
tool-set. Further, certain embodiments provide imaging for
Nanoelectronics, reliability and failure analysis in Microsystems
(MEMS), and Nanotechnology, in general, and especially biomolecular
interconnects and BioMEMS. Additionally, certain embodiments
provide in-vitro imaging of biological structures without having to
"open-up" internal structures. By combining the nanometer-scale
spatial resolution of conventional SPMs with the sub-surface
imaging capabilities, certain embodiments may characterize the
surface defects and structures with high resolution and will have
further potential for developing nanoscale non-invasive 3D
tomography, for example.
Scanning Near Field Ultrasound Holography (SNFUH) may be used, for
example, in near contact and contact mode with product frequencies
for the following structures and devices: (1) Investigating
mechanical uniformity and process-induced mechanical modification
of materials in integrated circuit (IC) structures and MEMS; (2)
Real-time in-vitro biological imaging of red blood cells infected
with malaria parasites; (3) Voiding in copper interconnects and (4)
Non-invasive monitoring of nanoparticles buried under polymeric
films. Such capabilities may complement cross-sectional imaging
techniques such as SEM-EDS (scanning electron microscope-energy
dispersive spectroscopy), TEM-EDS (transmission electron
microscope-energy dispersive spectroscopy), TEM-EELS (transmission
electron microscope-electron energy-loss microscopy), and ex situ
STM (scanning tunneling microscopy) to investigate the
nanomechanics and subsurface imaging of material interfaces, the
uniformity of conformally deposited coatings, and mechanical
defects in multilayer structures, for example.
Many other applications of the present invention as well as
modifications and variations are possible in light of the above
teachings. While the invention has been described with reference to
certain embodiments, it will be understood by those skilled in the
art that various changes may be made and equivalents may be
substituted without departing from the scope of the invention. In
addition, many modifications may be made to adapt a particular
situation or material to the teachings of the invention without
departing from its scope. Therefore, it is intended that the
invention not be limited to the particular embodiment disclosed,
but that the invention will include all embodiments falling within
the scope of the appended claims.
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